Optical receivers for the utilizing of digital coherent detection have recently been put into practical use. In digital coherent detection, the optical receiver recovers a transmission signal from a received optical signal by using local oscillation light. In this case, a frequency of the local oscillation light is nearly equal to a carrier frequency of an optical signal (that is, an optical frequency of a light source used in an optical transmitter). It is difficult, however, to make the carrier frequency precisely match the frequency of the local oscillation light. If the carrier frequency does not match the frequency of the local oscillation light, data cannot be recovered in the optical receiver in some cases. Thus, the optical receiver that receives an optical signal using digital coherent detection is provided with a circuit for estimating a difference between the carrier frequency and the frequency of the local oscillation light. The difference between the carrier frequency and the frequency of the local oscillation light is hereinafter referred to as “frequency offset”. Further, a frequency offset estimation circuit estimates or calculates the frequency offset.
The frequency offset estimation circuit detects a phase at every received symbol, for instance. A phase of each received sign is calculated based on an I-component and a Q-component of the received signal. The frequency offset estimation circuit calculates a difference between two successive symbols and estimates a frequency offset based on the phase difference.
The frequency offset estimation circuit includes a loop filter (or an averaging circuit) for averaging frequency offsets calculated in accordance with a phase shift between the symbols. Thus, an influence of noise occurring in an optical transmission path between the optical transmitter and the optical receiver is suppressed.
Note that a wireless communication apparatus provided with a frequency compensation function for correcting a phase error that is less than the quantitative step in a phase detection circuit has been proposed (Japanese Laid-open Patent Publication No. 9-232917, for instance). Also, a digital demodulator for making a carrier frequency error sufficiently small in a synchronous detection has been proposed (Japanese Laid-open Patent Publication No. 11-55338, for instance).
A large amount of noise may be added to an optical signal in a long-distance optical transmission system. In such a case, where the noise in a received optical signal is large, a phase error of each symbol detected by the frequency offset estimation circuit becomes large. Thus, the estimation accuracy of a frequency offset is reduced.
It may be possible to solve this problem by making a coefficient of the loop filter larger, for example. That is, if the coefficient of the loop filter is large, a noise tolerance is improved. However, when the coefficient of the loop filter is large, the performance for tracking optical frequency is low. Thus, it takes long time until the frequency offset is accurately estimated.